Base metal sulfide ores exist in many areas of the world and are a potential source of many value metals. In particular, the ores may contain zinc, nickel, copper, cobalt and the PGMs, silver and gold. The principal ores are all iron-bearing, and examples particularly include nickeliferous pyrrhotite Fe8S9, pentlandite (FeNi)9S8, chalcopyrite CuFeS2, arsenopyrite FeAsS and sphalerite ZnS. Cobalt may be found in the lattice of a pentlandite ore. Base metal sulfide ores have been used extensively in the commercial production of nickel, cobalt, zinc and copper.
Base metal sulfide ores may be processed using hydrometallurgical or pyrometallurgical techniques. Recovery of nickel, copper and PGMs tends to be high with the pyrometallurgical route, typically being greater than 90%, and cobalt recovery is typically between 30 and 70%. Recovery of nickel, cobalt, zinc and copper is also high in the hydrometallurgical route, but PGMs and gold tend to be lost in the leach residue unless further, often complicated and costly, recovery processes are carried out.
Smelting of nickel sulfide concentrates produces a liquid furnace matte. The liquid furnace matte is then subjected to air oxidation, in a process known as converting, to remove most of the iron and sulfur. Iron and gangue impurities are removed as a disposable slag. The resulting converter matte, also known simply as matte, may then be treated to obtain the nickel, cobalt, copper and PGMs and gold. The treatment methods used are mainly hydrometallurgical, for example refining processes based on sulfate, carbonyl, ammoniacal and chloride chemistry. Sulfate and especially chloride-based refining processes are discussed by G. Van Weert in “Some Observations on the Chloride Based Treatment of Nickel-Copper-Cobalt Mattes” pages 277-298 of Chloride Metallurgy 2002—Volume 1, 32nd Annual Hydrometallurgy Meeting, Edited by E. Peek and G. Van Weert, published by CIM.
In a chloride leach process, the most valuable component, viz. nickel, may be solubilized first, with little leaching of copper, thus achieving a separation of nickel from copper. In a known chloride leach process (Thornhill, P. G., Wigstol, E and Van Weert, G., “The Falconbridge Matte Leach Process”, Journal of Metals, 23(7), 1971 p13) using very strong hydrochloric acid, the leach may be represented as follows:Ni3S2+6HCl=>3NiCl2+2H2S+H2 
In an alternative leach process based on chlorine, a granulated converter matte is ground and fed to a chlorine leach process where it is subjected in a first step to a redox controlled leach process solubilizing most of the nickel and part of the copper, but none of the PGMs:Ni3S2+3Cl2=>3NiCl2+2S0 Cu2S+Cl2=>CuS+CuCl2 
To remove cupric copper, which is the regarded as the leachant, additional matte is added without chlorine, followed by cementation. In another alternative, advantages of a chlorine leach could be achieved using sub-azeotropic hydrochloric acid and oxygen. Solubilized copper (cupric) chloride would again be the leaching agent.
The hydrometallurgy of complex sulfide bulk concentrates is discussed by D. S. Flett in “Chloride Hydrometallurgy for Complex Sulfides: a Review” pages 255-276 of Chloride Metallurgy 2002—Volume 1 above. In particular, the ferric or cupric chloride leaching of Cu/Pb/Zn/Ag type sulfide concentrates is discussed. Recent activity in the treatment of single sulfide concentrates, particularly copper e.g. pressure leaching using BrCl2− as oxidant, is also reported. The article concludes that this is the most promising process for commercialization but the development of processing of complex sulfide concentrates still has some way to go before commercialization is finally realized.
A process for recovering non-ferrous metal values from a metal-containing sulfide material containing at least one of zinc, copper, lead, cobalt, nickel, silver and gold, as well as iron, is disclosed in U.S. Pat. No. 4,378,275 of Adamson et al, issued Mar. 29, 1983. The sulfide material is leached under oxidizing conditions with acidic aqueous chloride lixiviant solution containing magnesium chloride. The oxidizing conditions are disclosed as use of molecular oxygen in the form of air, oxygen-enriched air and pure oxygen. Although leaching at atmospheric pressure is stated to be possible, it is preferable to operate the leach stage under elevated partial pressures, i.e. under pressure leach conditions. Use of elevated temperatures is preferred, i.e. at least about 50° C. to about 250° C., with temperatures in the range of 100° C. to 180° C. being preferred. The period for leaching is from about 5 minutes to about 12 hours. The use of low chloride levels is preferred. For example, Adamson et al. provides that the chloride ion concentration is typically from about 4 to about 6 grams of ions per liter. The kinetics of the process would indicate a need to use long periods of leaching at the lower temperatures and atmospheric pressure. Pressure leaching, using oxygen, of a Zn/Cu/Fe ore containing very low levels of nickel at 160° C. is exemplified. In the process, non-ferrous metal values are solubilized, leaving iron oxide and sulfur as a residue. The leach liquor is subjected to liquid—liquid extraction using a hydrophobic extractant. The raffinate, containing magnesium chloride and any sulfates formed during the leach process, is subjected to pyrohydrolysis to yield hydrogen chloride and magnesium oxide. The sulfates are then removed by washing of the magnesium oxide formed, which counteracts many of the advantages of forming magnesium oxide by pyrohydrolysis.